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[2003:d8:2f0c:c200:fa4a:c4ff:1b32:21ce]) by smtp.gmail.com with ESMTPSA id ffacd0b85a97d-42708bcea1bsm1267844f8f.14.2025.10.17.01.43.52 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Fri, 17 Oct 2025 01:43:53 -0700 (PDT) Message-ID: <4a7a7fbb-e33e-4033-91e7-efce7915cf7f@redhat.com> Date: Fri, 17 Oct 2025 10:43:52 +0200 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH mm-new 1/1] mm/khugepaged: guard is_zero_pfn() calls with pte_present() To: Lance Yang , Wei Yang , Dev Jain Cc: akpm@linux-foundation.org, lorenzo.stoakes@oracle.com, ziy@nvidia.com, baolin.wang@linux.alibaba.com, Liam.Howlett@oracle.com, npache@redhat.com, ryan.roberts@arm.com, baohua@kernel.org, ioworker0@gmail.com, linux-kernel@vger.kernel.org, linux-mm@kvack.org References: <20251016033643.10848-1-lance.yang@linux.dev> <17c4c5f9-6ac8-4914-838f-f511dfbf948f@arm.com> <20251017012724.4bo5oj2g6tdmp2fv@master> <1674efca-6d4e-4247-8b1c-b6816360d8bb@redhat.com> From: David Hildenbrand Autocrypt: addr=david@redhat.com; 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charset=UTF-8; format=flowed Content-Transfer-Encoding: 8bit X-Stat-Signature: mqhr9cmbjjtoeyrgownfr1r7r5n3r4wk X-Rspamd-Queue-Id: 7FC15A0006 X-Rspamd-Server: rspam06 X-Rspam-User: X-HE-Tag: 1760690637-276416 X-HE-Meta: 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 49ZaJiFE b/fqX7Q/cM8F266Z4jXOCwxwKWx8KUxg08MkpajbCL/9CWKThXPkUTS9D3Z0u34cW3MpqI3QBOtSgXkJ9C6+sxOqlNMGCIMgfyXH5pRzHEiN8ZfP7jVse+E3NYOmd0dS7j7UJ8gqq2QfGdsAOfcVmzqVKQ9OnPXHnZ5rjCFb2fFo6gAO4rKZo5/7eZMobOQ9fvJbcE4YSW0oOHtA29v32+z+DFM+D1YUk0PdGGtF9+dzIyO6HctYVVcdb2eA8ycLZCRoKcFnBb6aHx4ZbZ8avJ+6dsuRCVT/mXkhFNBLpHLok46i1q9x1roY48wk91sta4uPzJPctNCHDXyXqtunrAjQIJO0Pe+Hc+YMX4TeZ5Vqc9iz4xf3ykGY78brKkzXTMV7xbXYA4V9s2ugAspocWl7Ft/kUOs5nVY4PzO2kc+M+QTyf7u+aNjXss/R1ZFi8A7s5z7/Txu0U9DnwdBiVHnbRDRkDFNCG/BQdWEGRuuxzMAgcUcnIt+GnOTUQKXAyO7IXTzyScCd6oaHV1OcNek64c9B1ACz7UaaqYn5P+GamzrBq/yT0Wvz/qpZWlH7REDlqDPAGNxLgejZbFi5Zwng1rA== X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On 17.10.25 10:37, Lance Yang wrote: > > > On 2025/10/17 16:11, David Hildenbrand wrote: >> On 17.10.25 03:27, Wei Yang wrote: >>> On Thu, Oct 16, 2025 at 11:47:06AM +0530, Dev Jain wrote: >>>> >>>> On 16/10/25 9:06 am, Lance Yang wrote: >>>>> From: Lance Yang >>>>> >>>>> A non-present entry, like a swap PTE, contains completely different >>>>> data >>>>> (swap type and offset). pte_pfn() doesn't know this, so if we feed it a >>>>> non-present entry, it will spit out a junk PFN. >>>>> >>>>> What if that junk PFN happens to match the zeropage's PFN by sheer >>>>> chance? While really unlikely, this would be really bad if it did. >>>>> >>>>> So, let's fix this potential bug by ensuring all calls to is_zero_pfn() >>>>> in khugepaged.c are properly guarded by a pte_present() check. >>>>> >>>>> Suggested-by: Lorenzo Stoakes >>>>> Signed-off-by: Lance Yang >>>>> --- >>>>>    mm/khugepaged.c | 13 ++++++++----- >>>>>    1 file changed, 8 insertions(+), 5 deletions(-) >>>>> >>>>> diff --git a/mm/khugepaged.c b/mm/khugepaged.c >>>>> index d635d821f611..0341c3d13e9e 100644 >>>>> --- a/mm/khugepaged.c >>>>> +++ b/mm/khugepaged.c >>>>> @@ -516,7 +516,7 @@ static void release_pte_pages(pte_t *pte, pte_t >>>>> *_pte, >>>>>            pte_t pteval = ptep_get(_pte); >>>>>            unsigned long pfn; >>>>> -        if (pte_none(pteval)) >>>>> +        if (!pte_present(pteval)) >>>>>                continue; >>>>>            pfn = pte_pfn(pteval); >>>>>            if (is_zero_pfn(pfn)) >>>>> @@ -690,9 +690,10 @@ static void >>>>> __collapse_huge_page_copy_succeeded(pte_t *pte, >>>>>             address += nr_ptes * PAGE_SIZE) { >>>>>            nr_ptes = 1; >>>>>            pteval = ptep_get(_pte); >>>>> -        if (pte_none(pteval) || is_zero_pfn(pte_pfn(pteval))) { >>>>> +        if (pte_none(pteval) || >>>>> +            (pte_present(pteval) && is_zero_pfn(pte_pfn(pteval)))) { >>>>>                add_mm_counter(vma->vm_mm, MM_ANONPAGES, 1); >>>>> -            if (is_zero_pfn(pte_pfn(pteval))) { >>>>> +            if (!pte_none(pteval)) { >>>> >>>> Could save a level of indentation by saying >>>> if (pte_none(pteval)) >>>>     continue; >>>> >>> >>> Vote for this :-) >> >> I suspect there will be a v2, correct? > > I was hoping a v2 wouldn't be necessary for this ;p > > Of course, if we'd prefer a v2, I'm happy to send one out. I lost track of what the result will be, so a v2 would be nice at least for me :) -- Cheers David / dhildenb